The invention relates generally to the manufacture of electronic devices. More specifically, this invention relates to methods of trimming photoresist patterns useful in shrink processes for the formation of fine lithographic patterns.
In the semiconductor manufacturing industry, photoresist materials are used for transferring an image to one or more underlying layers, such as metal, semiconductor and dielectric layers, disposed on a semiconductor substrate, as well as to the substrate itself. Photoresist materials further find use in semiconductor manufacture in the formation of ion implantation masks. To increase the integration density of semiconductor devices and allow for the formation of structures having dimensions in the nanometer range, photoresists and photolithography processing tools having high-resolution capabilities have been and continue to be developed.
Positive-tone chemically amplified photoresists are conventionally used for high-resolution processing. Such resists typically employ a resin having acid-labile leaving groups and a photoacid generator. Exposure to actinic radiation causes the acid generator to form an acid which, during post-exposure baking, causes cleavage of the acid-labile groups in the resin. This creates a difference in solubility characteristics between exposed and unexposed regions of the resist in an aqueous alkaline developer solution. In a positive tone development (PTD) process, exposed regions of the resist are soluble in the aqueous alkaline developer and are removed from the substrate surface, whereas unexposed regions, which are insoluble in the developer, remain after development to form a positive image.
One approach to achieving nm-scale feature sizes in semiconductor devices is the use of short wavelengths of radiation such as 200 nm or less, for example, 193 nm or EUV wavelengths (e.g., 13.5 nm) during exposure of chemically amplified photoresists. To further improve lithographic performance, immersion lithography tools have been developed to effectively increase the numerical aperture (NA) of the lens of the imaging device, for example, a scanner having a KrF or ArF light source. This is accomplished by use of a relatively high refractive index fluid (i.e., an immersion fluid) between the last surface of the imaging device and the upper surface of the semiconductor wafer. The immersion fluid allows a greater amount of light to be focused into the resist layer than would occur with an air or inert gas medium. When using water as the immersion fluid, the maximum numerical aperture can be increased, for example, from 1.2 to 1.35. With such an increase in numerical aperture, it is possible to achieve a 40 nm half-pitch resolution in a single exposure process, thus allowing for improved design shrink. This standard immersion lithography process, however, is generally not suitable for manufacture of devices requiring greater resolution, for example, for the 32 nm and 22 nm half-pitch nodes.
Considerable effort has been made to extend the practical resolution beyond that achieved with standard photolithographic techniques from both a materials and processing standpoint. For example, multiple (i.e., double or higher order) patterning processes have been proposed for printing CDs and pitches beyond lower resolution limits of conventional lithographic tools. One such double patterning process is litho-litho-etch (LLE) double patterning, which involves formation of a first lithographic photoresist pattern followed by formation of a second lithographic photoresist pattern, wherein lines of the second pattern are disposed between adjacent lines of the first pattern. Such a process is disclosed, for example, in U.S. Patent Application Publication No. US2008/0063985A1. LLE double patterning and other advanced lithographic processes often require the formation of isolated features such as lines or posts by direct lithographic printing. The formation of isolated features with an acceptable process window, however, can pose a challenge as a result of poor aerial image contrast at defocus.
There is a continuing need in the art for improved photolithographic methods for the formation of fine patterns in electronic device fabrication.